The present application relates generally to physiological monitoring. It finds particular application in conjunction with pulse oximetry and will be described with particular reference thereto. However, it is to be understood that it also finds application in other photoplethysmographic measurement scenarios and is not necessarily limited to the aforementioned application.
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin. A sensor is placed on a thin part of the patient's body. In the case of an infant, the sensor is usually placed across a foot. Otherwise, the sensor is usually placed across a fingertip or an earlobe, or alternatively on the forehead. Light at red (e.g., around 660 nm) and infrared (e.g., around 940 nm) wavelengths is then passed sequentially through the patient to a photo-detector. The changing absorbance at each of the two wavelengths is measured to create a photoplethysmogram (PPG) for each of red light and infrared light.
Based on the PPGs, the absorbance due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, fat, fingernail polish (in most cases), and so on, can be determined. Using the ratio of changing absorbance of the red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen-unbound (dark red or blue in severe cases) blood hemoglobin, a determination of oxygen saturation (SpO2) can be made. Oxygen saturation is the percentage of hemoglobin molecules bound with oxygen molecules to total hemoglobin (oxygenated and non-oxygenated).
Since pulse oximetry is non-invasive and convenient to use, it has become common place within hospital settings for patient monitoring, as well as in home healthcare settings. However, there are a number of conditions, due to the nature of photoplethysmographic measurement, which reduce the accuracy of the SpO2 calculation. This is also true for the calculations of percent saturated carboxyhemoglobin (SpCO), pulse rate (PR), pulse pressure variation (PPV), pulse transient time (PPT), hemoglobin (HB), hemocrit (HCT), glucose (GLU), cholesterol (CHOL), and any other physiological parameters derived by using photoplethysmographic measurements from the PPGs. This reduced accuracy, in turn, increases the number of false alarms.
One condition which reduces accuracy is low-perfusion (i.e., hypo-perfusion). Low-perfusion can, for example, cause falsely low SpO2 readings. However, this condition can be identified by analyzing the PPG signal itself so long as the PPG signal is clean (i.e., free of noise and/or artifacts). The user can then be notified of the condition, so the user knows that current measurements are inaccurate.
Another condition which reduces accuracy is movement of the sensor. When there is a movement with the sensor, the measuring condition is disturbed (i.e., the relationship between the absorbance from red and infrared photo sensors is no longer in the theoretically logical situation), so that the measurements from the movement period are no longer reliable and thus should not be used for, for example, display and/or alarming purpose to indicate one or more of SpO2, SpCO, PR, PPV, PPT, HB, HCT, GLU, and CHOL of the patient.
To compensate for motion artifacts requires identification of movement of the sensor and identification of those portions of the PPG signal associated with motion artifacts. One way to identify motion artifacts is to analyze the PPG signal and find atypical patterns. However, this is a challenging task because PPG patterns associated with patho-physiological situations and with motion artifacts can both vary over a wide range and overlap. Namely, the time-domain and/or frequency-domain features of the noise and/or artifacts associated with the sensor movement in the PPG signal can appear similar to those of the PPG itself. Further, the PPG signal processing techniques used for identifying motion artifacts in PPG signals are often complicated and require certain operational controls, such as “Learning Period” and “Re-Learn” action.
The present application provides a new and improved system and method which overcome the above-referenced problems and others.